As a technique for inhibiting gene expression, for example, RNA interference (RNAi) is known (Non-Patent Document 1). Inhibition of gene expression by RNA interference is generally carried out, for example, by administering a short double-stranded RNA molecule to a cell or the like. The aforementioned double-stranded RNA molecule is generally called siRNA (small interfering RNA). It has been reported that gene expression can also be inhibited by a circular RNA molecule having a double strand partially formed therein by intramolecular annealing (Patent Document 1). However, in these techniques, the RNA molecules to induce the inhibition of the gene expression have the following problems.
First, in order to produce the aforementioned siRNA, it is necessary to synthesize a sense strand and an antisense strand separately and to hybridize these strands at the end of the process. Thus, there is a problem of low manufacturing efficiency. Furthermore, when the aforementioned siRNA is administered to a cell, it is necessary to administer the siRNA to the cell while repressing the dissociation to single-stranded RNAs, which requires a laborious task of setting the conditions for handling the siRNA. The circular RNA molecule has a problem in that its synthesis is difficult. To deal with the situation, the present inventors constructed a new single-stranded nucleic acid molecule having a two-loop structure produced by self-annealing, which solves the problem (patent document 2).